1. Field of the Invention
This invention generally relates to x-ray imaging and more specifically to apparatus for enhancing a cephalometric image in a dental x-ray.
2. Description of Related Art
In dentistry and other medical disciplines diagnoses and treatments can be aided by x-ray imaging of the skull. This invention is particularly directed to cephalometric imaging for obtaining a lateral view of the skull by means of transmissive radiographic imaging directed transversely across the median plane. Generally speaking, such an image is obtained by positioning a patient's head between an x-ray source and a film cassette. The x-ray energy is then directed toward the patient's head to the cassette to produce a full image of the skull on the film.
However, given the wide range of densities of the skeletal and soft tissue portions of the head, a conventional x-ray will either properly expose the skeletal portions of the head and over-expose the soft tissue portions or properly expose the soft tissue portions and underexpose the skeletal portions. As known, this result occurs because the areas of different density in the head will attenuate the x-ray energy differently. Normally a technician will adjust the exposure to expose the skeletal portions properly. Consequently, the resulting image will be either devoid of any soft tissue information or have only minimal information that will not be easily discerned by a visual inspection of a film image.
A number of approaches have been taken in an attempt to overcome this problem so that a single x-ray exposure will provide good image detail of both skeletal and soft tissue areas of a patient's head. One approach involves interposing a wedge-shaped shield for attenuating the intensity of the x-ray passing through an area dominated by soft tissue. In some cases these shields, or "soft tissue shields," are shaped metal devices that attach to the exterior of the film cassette. For cephalometric imaging, the soft tissue shield mounts to extend vertically across the front end of the cassette, that is, the end of the cassette aligned with the frontal area of the patient's head including the forehead and chin areas.
U.S. Pat. No. 5,454,023 (1995) to Asikainen depicts a cephalostat with a soft tissue shield, or filter, that is interposed between the x-ray source and the patient. This cephalostat uses the filter to attenuate the intensity of the x-rays before they reach the soft tissue areas of the patient.
U.S. Pat. No. 5,058,147 (1991) to Nishikawa et al. depicts another embodiment of dental x-ray apparatus for providing panoramic or cephalometric images. This apparatus includes a first cassette holder for a cephalometric cassette and a second cassette holder for a panoramic cassette. A control mechanism moves an x-ray generator between two positions depending upon the type of image that has been selected. In addition, the control system alters radiation slits on the x-ray generator to alter the characteristic x-ray energy emitted from the generator for the two different image types.
Still another approach utilizes specially constructed film cassettes. Normal film cassettes provide an image on x-ray film by placing the film having one emulsion side against one conversion screen or having two emulsion sides between two conversion screens. The conversion screens convert impinging x-radiation into energy having a wavelength that corresponds to a sensitive film wavelength. For example, if the film is sensitive to blue light, the conversion screen will be formed to produce blue light in response to x-ray radiation with the intensity of the light being a function of the x-ray intensity.
Generally a conversion screen has a uniform response characteristic. That is, if the screen is subjected to a uniform radiation intensity, it will produce an even exposure on the film adjacent to it. Specially constructed conversion screens alter the response characteristic over different areas of the screen. In one such system, the conversion of areas that will image predominantly skeletal portions convert a greater portion of the x-ray radiation to light than the areas that will image predominantly soft tissue. Consequently, the soft tissue areas will be exposed less than the skeletal areas.
U.S. Pat. No. 5,734,693 (1998) to Quint et al. depicts a radiation exposure system that incorporates such specially constructed conversion screens and optical filters in a film cassette. X-ray radiation received from the patient passes through a front face of the cassette, a first conversion screen, an optical filter, the x-ray film and a second screen. The filter has an area of a first optical density of a first material and a second area of a second optical density. In the specific embodiment, the second area is formed by removing corresponding portions of the first material. This patent also suggests using an optical filter formed of a partially radiation opaque, absorbing or blocking material thereby to control the exposure of the second screen. In this apparatus, at least one of the screen layers includes an area of different amounts or reactant material, a selective grading from a front end of the screen to the back end being described.
In still another approach, the x-rays from the patient energize a photoarray connected to a computer system for scanning an x-ray image. With such an image it is possible to enhance portions of the image so that soft tissue appears in the final image.
Each of the foregoing approaches have certain drawbacks. Specially designed equipment, such as described n the Asikainen and Nishikawa et al. patents, is much more expensive than conventional dental x-ray machines. The nature of many dental practices can not justify the expense of obtaining such special equipment. Such practices have sought and do seek to obtain good cephalometric imaging by the use of special cassettes. However, such special cassettes are also expensive and are limited in application. This requires a larger inventory of film cassettes and the development of procedures for assuring that, for a given x-ray, the proper cassette is selected.